1. Field of the Invention
This invention relates to a method for cleaning and disinfecting optical contact lenses by the application of ultrasonic energy.
2. Description of the Prior Art
Optical contact lenses enjoy increasing popularity among persons requiring vision correction. The first contact lenses were made of glass; later plastics, principally polymeric methacrylates, replaced glass as the material for forming so-called "hard" contact lenses. More recently, hydrophilic copolymers have been used to produce so-called "soft" contact lenses.
A contact lens rests on the surface of the cornea, which is covered with a natural fluid layer of tears, oil and other glandular secretions that constantly bathe and moisten the eye. It is important, therefore, that contact lenses be kept free of bacteria and soil that may be harmful to the eye tissue.
Elaborate procedures, often involving the use of expensive equipment and chemicals, have been devised to enable the user of contact lenses to disinfect and/or clean the lenses on a daily basis in order to prevent damage to or infection of the eye. The term "disinfect" or "disinfection" as used herein means to destroy pathogens (or disease-carrying organisms). The term "clean" or "cleaning" as used herein means the removal of separable foreign matter such as dirt, dust and particularly secretions from the eye including the protein lysozyme. While there is no specific requirement to disinfect hard contact lenses, disinfection has been found to be highly desirable.
Because of their hydrophilic properties, soft contact lenses pose a considerable challenge to disinfection. Disinfection of soft contact lens material was first carried out by placing the lenses into a container which was then placed in a boiling water bath for 10 or 15 minutes. The unit devised for this procedure was essentially a double boiler. Continued boiling at 100.degree. C. had a tendency to cause the lens material to degrade; more importantly, however, the high temperature caused the protein material present on the surface of the lens to denature and become "baked" on the lens surface.
Thereafter, chemical disinfection systems were devised to replace the heating units; these systems required at least two and sometimes three separate solutions to accomplish disinfection, but enjoyed some early acceptance because of their convenience over the use of heating units. The incidence of "red eye," a condition of eye irritation that may be caused by preservatives present in the solutions, prompted many soft contact lens uses to revert to heat disinfection systems.
The next improvement in contact lens disinfection was the development of a dry heater wherein the heat source was a dry heat source, essentially a metal plate with a resistance heater. However, the temperature of the plate was difficult to control; therefore, there was still presented a danger of overheating the lens and baking protein onto the surface of the lens. Rather than using a metal plate, some units contained paraffin which surrounded the cavity in which the lens case was placed. The paraffin was heated to a liquid state. Because paraffin has a lower specific heat than metal, the paraffin retained heat at a more uniform level than the metal. The lens deterioration problem was alleviated, but the protein denaturation and baking problems remained.
All of the units which disinfect contact lenses by heating them are required, by present government regulation, to maintain the lenses in an environment having a minimum temperature of 80.degree. C. for a minimum time period of ten minutes. Most units include a safety factor so that in reality the temperature reaches 90.degree. C. or even 100.degree. C. for at least a portion of that time period. At these temperatures any protein which remains on the surface of the lens will denature and be baked on.
Throughout the evolution of disinfection techniques for soft contact lenses, the almost universal technique for cleaning the lenses was a manual one, in which the user applied a cleaning solution to each lens and rubbed it between his fingers. This practice, carried out either before or after the separate disinfection step, physically removed soil and debris from the lens. The same manual cleaning procedure long has been used with hard contact lenses. The problem in using the manual cleaning procedure for soft contact lenses is that the lens is likely to tear or be otherwise damaged if the lens is rubbed vigorously enough to remove all of the protein. But, if the lens is not rubbed vigorously enough, some of the protein remains on the lens and is denatured and baked on when the lens is placed in a heating unit for disinfection. One type of chemical cleaning system involved the uses of enzymes. The purpose of the enzymes was to remove the protein from the surface of the lens by breaking it down. However, once protein build up occurred on the surface of the lens, the enzymes were largely ineffective.
Ultrasonic cleaning techniques were developed in response to the problems associated with the aforementioned cleaning procedures. In this way the lens can be cleaned without the chance of damaging the lens. Ultrasonic cleaning techniques generally involve placing the contact lenses into a holder and immersing the holder in a cleaning fluid bath. The holder is adapted so that the cleaning fluid can circulate around the lenses. The ultrasonic energy causes a phenomenon known as cavitation to occur which cleans the foreign matter from the surface of the lenses.
Cavitation occurs in liquids exposed to periodic oscillatory forces and is usually explained as the formation and rapid collapse of small cavities in the liquid. The collapse of these cavities produces large amplitude shock waves and elevated local temperatures. Electrical discharges are also believed to occur during the collapsing phase. For further discussion of cavitation in liquids produced by ultrasonic energy, see e.g. U.S. Pat. Nos. 3,837,805; 3,481,687; and 4,086,057; and "Ultrasonics" by R. M. G. Boucher, Canadian Journal of Pharmaceutical Sciences, Vol. 14, No. 1, 1979, pp. 1-12.
In the known applications of ultrasonic energy to contact lens cleaning and disinfection, the cleaning bath is heated prior to introducing the lenses into the bath. Generally, the cleaning bath is maintained at a temperature of at least 65.degree. C. throughout the cleaning and disinfection processes. Thus the same problems of protein denaturation and degradation of the lens material occur as in other thermal disinfection units due to the use of such a high temperature. However, a synergistic effect between the ultrasonic energy and the temperature of the cleaning bath is observed so that disinfection does occur in the cleaning bath at temperatures lower than the temperatures which would be required without the application of the ultrasonic energy to the cleaning bath. For further discussion of the synergistic effect between the temperature of the bath and the application of ultrasonic energy to the bath, see U.S. Pat. Nos. 3,837,805 and 4,211,744.
West German patent application No. 24 38 067 describes an ultrasonic cleaning and disinfection unit for contact lenses in which the lenses are cleaned and disinfected at a temperature below 70.degree. C. and preferably below 55.degree. C. Ultrasonic energy is transmitted through a biologically inert, nonlethal solution containing the lenses at a frequency of 55 kHz at a power level of between 3 and 5 watts. A time period of between 11/2 and 4 hours is required before the lenses are disinfected, thus the procedure is impractical when a short cleaning and disinfection period is required. The unit contains temperature control means to shut off the source of ultrasonic energy when the temperature rises more than a few degrees above the desired level and reactivates the source of ultrasonic energy when the temperature falls more than a few degrees below the desired level.